In modern public land mobile networks (PLMN) based on a cellular network, the system consists in a known way of several mobile stations (MS) using the system, such as mobile telephones, and a fixed base station subsystem (BSS). This base station subsystem comprises usually several base transceiver stations (BTS) distributed in a geographical area, and each base transceiver station serves a cell comprising at least part of this geographical area. Data transmission, e.g. transmission and reception of speech or data, between a mobile station located in the area of the cell and the base transceiver station, takes place by means of radio communication by using radio channels reserved for the cell. The operation of one or several base stations is controlled by a base station controller (BSC) belonging to the base station subsystem and having the function of e.g. controlling the use of radio channels. The area of the cell served by the base station depends inter alia on the strength of the transmitter of the base station, wherein the sufficient strength for the signal of the radio channel received by the mobile station depends e.g. on the distance from the mobile station to the base station. The strength of the signal received by the mobile station is influenced for example by the topography of the geographical area of the cell and the directivity of the antenna of the base station, even though the distance from the mobile station to the base station remains the same. Consequently, the shape of the coverage area of the cell varies to a great extent. However, when illustrating the operating principle of a cellular network, usually either circular or hexagonal areas are used to illustrate a cell. One or several base station controllers communicate with a mobile services switching centre (MSC) which transmits the communications between base stations belonging to the mobile services switching centre within the mobile communication network, to other mobile services switching centres, or to a public switched telephone network (PSTN).
When a mobile station, such as a mobile phone or a mobile terminal placed in a vehicle, moves from one cell to another, there is an arrangement in public mobile communication networks for preventing disconnection of ongoing data transmission e.g. between a first base station and the mobile station during this move. The data transmission in question can be for example a telephone conversation between two users of mobile phones or a data transmission between a mobile terminal and a computer coupled with a public switched telephone network. Disconnection of a radio connection used for data transmission can result from e.g. the fact that the signal of the radio channel of the base station serving the first cell is weak and/or is lower than the level below which reliable and error-free data transmission is not possible. Thus the data transmission must be transferred to the radio channel used by the base station serving another cell. This transfer can be implemented by these two base stations and a base station controller, if they are coupled to the same base station controller, or further one or several mobile services switching centres if the base stations are coupled to different base station controllers. For implementing the transfer, the base station transmits inter alia a command to the mobile station to carry out a change of the radio channel used for the data transmission.
Because of the popularity of use of mobile stations, such as mobile terminals and particularly personal portable mobile phones, their number has continuously increased, and as a result, the charges paid by a mobile subscriber for data transmission services have come down because of the competition between different network operators. Furthermore, network operators endeavour to develop and increase the services offered by them to the users of mobile stations. These services include providing data transmission connections to a packet data network (PDN) and to an integrated services digital network (ISDN) in addition to a public switched telephone network. By means of these data transmission connections, it is also possible e.g. for different service providers of the INTERNET network to develop their services for the users of mobile stations, such as mobile phones and portable computers connected with them e.g. by means of PCMCIA cards. Further, the network operators endeavour to offer mobile subscribers individual services, such as data transmission connections at a lower charge than the normal charge. Less expensive services can be offered e.g. in the area of the geographical location of the place of employment of the mobile subscriber, when the subscriber's mobile station communicates with the base station of this geographical area, i.e. the so-called office cell. Thus the charges debited for data transmission connections can be e.g. less expensive within the area of this office cell and normal in the area of other cells. It is also possible to offer less expensive services in the area of the cell with the geographical location of the mobile subscriber's home, i.e. the so-called home cell.
U.S. Pat. No. 5,568,153 discloses a system and method for maintaining data of mobile subscriber information based on defining the so-called home area and work area by means of the geographical location and a radius selected for the area. These data are entered in the home location register (HLR) of the mobile communication network, and on the basis of these information, the mobile services switching centre determines whether the mobile station is in the area of a cell served by a base station which is located in the geographically defined home area or work area. On the basis of this information, it is possible to change the charge to be debited for the data transmission connection when the mobile station moves to the service area of said base station. However, a considerable disadvantage of this system is the increased need of the mobile services switching centre to handle mobile subscriber registration data. Further, when there are several home and work areas and an increased number of service providers and services, there is also an increased number of registration data stored in the databases of the mobile communication network, such as the home location register and thus also the visitor location register (VLR). Furthermore, a reduction in cell areas cause an increased need for data transmission between registers e.g. in order to conduct channel changes particularly in densely populated urban areas. The shrinking of cells will result in an increase in registration data, which will also cause a need to raise the storage capacity of base stations and, further, the implementation will cause extensive changes e.g. in the controller software of the base station system. Moreover, the geographical location of the mobile station must be determined on the basis of the signal transmitted from the mobile station and received by e.g. by different base stations, which will further greatly increase the need for data transmission and cause delays in the mobile communication network. Further, U.S. Pat. No. 5,568,153 presented above does not disclose how to take into account with which serving base station the mobile station is in radio communication when located within the home area, but the charge is based solely in the geographical location of the mobile station.
In public mobile communication networks in practice, such as the Global System for Mobile Communication (GSM) standardised by the European Telecommunications Standards Institute (ETSI), the cells can be geographically at least partly overlapping. Further, it is possible that within the coverage area of the cell there is another, smaller cell served by a base station offering the client e.g. less expensive subscriber-specific services. Also thus in situations where the signal of the radio channel of the base station received by the mobile station particularly in the edge zones of the smaller cell has low strength, according to the prior art the data transmission must be transferred to be conducted between the mobile station and that base station where the signal of the radio channel has preferably the highest strength as measured by the mobile station.
According to prior art, when the mobile station moves to the coverage area of the smaller cell, it is also possible that the communication between the mobile station and the high-strength base station will not be transferred at all to the radio channel used by the base station of the smaller cell, because priority in cell selection is given advantageously to the radio channel used by the base station with the highest strength. The above-presented patent U.S. Pat. No. 5,568,153 does not disclose any alternatives for solving this problem. Thus in the placement of base stations, upon the stage of constructing a mobile communication network and upon reduction of the coverage areas of cells, one must increasingly take into account also the strength of signals of radio channels of base stations serving adjacent cells and cells operating in the same area in a single cell. This will raise the costs of planning and limit the placement of base stations as well as the options for selecting output power of base stations serving new cells.